The present invention relates to a vapor phase reflow system comprising a soldering zone and a process zone, a primary liquid reservoir containing a primary liquid, the vapor phase reflow system further comprising a heating means for converting the primary liquid into the vapor phase.
As is generally known, in the case of vapor phase reflow upon condensation of a saturated vapor phase, energy is liberated in the form of heat on the surface of a component to be soldered, whereby the solder is molten and the component is thus connected to the conducting track. The saturated vapor phase is produced by a boiling primary liquid, with the process temperature being predetermined by the boiling point of the primary liquid during soldering. Although the reflow or soldering process with saturated vapor is characterized by an exactly defined soldering temperature with uniform heating of the component to be soldered and is thus particularly well suited for processing components having different heating capacities, this method is somewhat limited since the soldering temperature cannot be freely chosen in an easy way as the temperature is actually predetermined by the very boiling point of the primary liquid.
An improvement in this respect is achieved with systems in which the vapor does not directly interact with its liquid phase any more. Small amounts of the primary liquid are here completely evaporated in the soldering space of the system, whereby amounts of vapor that are needed very quickly can continuously be produced and the available amount of heat can thus also be controlled to some extent. Nevertheless, this principle is also based on the measure that the boiling temperature of the primary liquid always forms an upper limit temperature, whereby a possibly desired temperature profile throughout the soldering operation can only be achieved through the residence times of the subassemblies to be soldered in the vapor. As a result, either the throughput of such a system is considerably reduced because of the increased residence time, or it sometimes happens that certain subassemblies which require a special type of temperature profile cannot be processed at all.
Furthermore, the prior art also reports on vapor phase reflow systems which employ superheated vapor so as to control the process temperature. For instance, GB-2 190 687 discloses a method and an apparatus in which superheated vapor is provided in the soldering zone by means of a suitable heating means, with the superheated vapor condensing at a certain level by means of cooling elements and being returned via a filter into a liquid reservoir for the primary liquid. In this known system, however, a complicated temperature profile cannot be adjusted together with a high throughput.
It is therefore the object of the present invention to provide an improved vapor phase reflow system in which a necessary temperature profile can be adjusted for the soldering process together with a high throughput of subassemblies to be soldered.
The present invention is characterized in that in the above-mentioned vapor phase reflow system primary liquid or the vapor thereof or both can be introduced by means of a primary-liquid fluid line into a soldering-zone heating module in fluid communication with the soldering zone and into a process-zone heating module in fluid communication with the process zone, the heating capacities of the soldering-zone heating module and of the process-zone heating module being adjustable independently of each other for generating superheated vapor based on the zones.
According to the present invention the primary liquid is heated by the heating means or converted into the vapor phase, which can take place at an adjustable or fixedly predetermined pressure and can then be passed to the various heating modules by means of the primary-liquid fluid line. In the heating modules the vapor and/or the primary liquid can be superheated to a freely selectable temperature by controlling the heating capacity of the vapor and/or the primary liquid. The superheated vapor can then be passed by means of nozzles or by means of convection onto the subassemblies to be processed into the respective zones. Hence, the process zone, which can e.g. serve as a preheating zone or second soldering zone, as well as the soldering zone can be operated with the vapor of the same primary liquid. This minimizes the operating costs of such a system, with the zone temperatures being freely selectable at the same time and permitting an individual adjustment of the soldering profile, as is standard in common convection systems. Thus the system of the invention makes it possible to achieve an individual temperature profile of the reflow type soldering process without the throughput of the system primarily depending on the residence time of the subassemblies to be processed within a condensation zone.
In a further embodiment of the present invention the primary-liquid fluid line comprises respectively controllable valve elements so that the amount of primary liquid or its vapor or both can be introduced in a controllable manner into the soldering-zone heating module and the process-zone heating module.
It is possible with this embodiment to control the amount of the superheated vapor in addition to the temperature of the superheated vapor generated in the respective heating modules and to thereby meet the demand for superheated vapor individually for different subassemblies in the soldering zone and the process zone, respectively. This results in a further reduction of the residence time because e.g. in the case of subassemblies comprising components of a relatively high heating capacity, it is possible to increase the amount of heat to be transferred per time unit by increasing the supplied amount of superheated vapor.
In a further embodiment the vapor phase reflow system comprises a respective fluid connection for discharging primary liquid and its vapor, respectively, so that the discharged primary liquid or the vapor thereof can be returned into the primary liquid reservoir and/or into the heating element for further vapor generation.
Since the vapor phase reflow system according to the invention can be operated with only one primary liquid, the primary liquid condensed in the process zone or soldering zone can be returned in a common line to the primary liquid reservoir or the heating element acting as a vapor generator. Advantageously, a corresponding filter element is provided in this fluid connection for purifying the returned and condensed primary fluid.
In a further embodiment the heating means for evaporating primary liquid is composed of partial heating elements and at least one partial heating element is provided in the soldering-zone heating module.
Thus, thanks to the modular structure of the heating element, primary liquid can be evaporated in addition and rapidly in the soldering-zone heating module upon demand and can then be superheated. To this end the primary-liquid fluid line suitably comprises a connection to the primary liquid reservoir so that primary liquid is permanently available in the soldering-zone heating module.
In a further development of the present invention a further partial heating element is provided in the process zone heating module.
When a partial heating element for evaporating primary liquid is provided in both the process-zone heating module and the soldering-zone heating module, a joint heating module for evaporating primary liquid can be completely omitted and the primary-liquid fluid line can be designed such that only primary liquid is transported to the process-zone heating module and the soldering-zone heating module. This results in a simple construction of the line system and reduces the overall losses of the system because the losses created during transportation of the vapor are avoided.
In a further embodiment of the present invention at least one second process zone is provided, the process zone serving as a preheating zone and the second process zone as a cooling zone, and the second process zone being in fluid communication with a second process-zone heating module into which primary liquid or vapor or both can be introduced via the primary-liquid fluid line.
Thanks to the provision of at least one second process zone, which is again equipped with an associated heating-zone module, complex heating profiles can be achieved for the soldering process. For instance, an individual temperature profile is possible within a wide range owing to the provision of two process zones with respectively associated process-zone heating modules, which serve as preheating zones, of a soldering zone and of a further process zone, which serves as a cooling zone. Furthermore, it is also possible to provide further process zones in combination with the process zone, the second process zone and the soldering zones, the further process zones being not operable by means of superheated vapor, but comprising conventional convection and/or infrared heating modules.
According to a further aspect the present invention refers to a method for operating a vapor phase reflow system, with a primary liquid being evaporated by means of a heating element, and the method being characterized in that the vapor of the primary liquid is superheated zonewise and in a controllable manner in a soldering-zone heating module in fluid communication with a soldering zone and in a process-zone heating module in fluid communication with a process zone.
In a further embodiment the temperature in the process zone and in the soldering zone is adjusted by controlling the amount of the introduced superheated vapor and/or the temperature of the superheated vapor.
The advantages of the method according to the invention have already been indicated in connection with the embodiments of the apparatus according to the invention.